Higher operating temperatures of gas turbine engines are continuously sought in order to increase the operating efficiency of such engines. As operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase, particularly the blades, vanes and shrouds of the high pressure turbine section. In addition to or as an alternative to increasing the high temperature capability of the material from which such components are formed, a common solution is to form a thermal insulating layer on the surfaces of the components in order to minimize their service temperatures.
For this purpose, thermal barrier coatings (TBC) formed directly on the component surface have found wide use. Such coatings generally entail the deposition of a metallic bond layer onto the surface of a component, followed by a ceramic layer which serves to thermally insulate the component. Preferably, the metallic bond layer is formed from an oxidation-resistant alloy, such as platinum aluminide, in order to promote the adhesion of the ceramic layer to the component, and thereby extend the life of the coating. Various ceramic materials have been employed as the ceramic layer, particularly zirconia (ZrO.sub.2) stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) stabilized by yttria, or other oxides. These materials can be readily deposited by physical vapor deposition (PVD) and air plasma spray techniques. These techniques require tooling to position, rotate and mask a component being coated, such that the coating process can be controlled to shield or coat selected portions of the component.
Because the equipment, tooling and maskants employed in the deposition process tend to become coated with the thermal barrier coating material, the coating material must be periodically removed from them in order to ensure their proper function and operation. For example, the removal of unwanted thermal barrier coating from a maskant is generally prudent when the thickness of the coating on the maskant is about 0.5 millimeter, which can develop in only a few coating cycles. The removal of coating from other tooling and equipment may be delayed until a coating thickness of about five to about eight millimeters is accumulated.
There is a corresponding need to repair the thermal barrier coatings of high temperature components of gas turbine engines in the event of spalling or unacceptable degradation of the coating. Repair methods generally entail completely removing the thermal barrier coating, restoring or repairing the surface of the component if necessary, and then recoating the component.
To be sufficiently resistant to impact damage and spalling, thermal barrier coatings preferably have a columnar structure that is very dense and hard. The columnar structure assists in relieving stresses created by the differential thermal expansions of the coating and the underlying substrate, and therefore reduces the tendency for spalling and the degree to which spalling will occur. Consequently, thermal barrier coatings are very difficult to remove while the component is in service and also during refurbishment of the component.
Prior art methods for removing thermal barrier coatings from engine components and coating equipment generally involve grit blasting the coating or subjecting the coating to an alkaline solution at high temperatures and pressures. However, grit blasting is a slow, labor-intensive process and erodes the surface beneath the coating. With repetitive use, the grit blasting process will eventually destroy the component or equipment. The use of an alkaline solution to remove a thermal barrier coating is also less than ideal, since the process requires the use of an autoclave operating at high temperatures and pressures.
Accordingly, it would be desirable to provide a method for removing a thermal barrier coating from components and coating equipment, in which the method does not damage the surface underlying the coating, does not require specialized and expensive equipment, and requires minimal labor to oversee the process and physically remove the coating.